A demand of an optical element constituting optical communication or optical storage has been increased with the spread of them. Particularly, in the optical communication, usage from a so-called backbone system centering on an optical fiber to a metro or access system is accelerated with practical application of wavelength multiplex transmission. For this reason, in the optical element used for add-drop and the like of an optical signal, usage of a small-sized and easily-integrated planar waveguide using a thin film of an optical material also proceeds in place of one in which a bulk material is combined.
In order to control light actively like modulation or switching of an optical signal, it is required to use a physics effect by interaction between an external input signal, such as electricity or heat, and a material that forms the optical element. In a silicon-system planar waveguide, an optical switch that uses a thermooptical effect by adding a heater to a directional coupler, a switch by combination of a MEMS (Micro Electric Mechanical System) and the like are known. However, both of them have a defect that response speed is slow in a μs (micro seconds) level. In addition, an optical switch using a thermooptical effect has a defect that power consumption is large while a switch of a MEMS type has a defect that a structure becomes complex and it is expensive.
An electro-optic element whose index of refraction is changed by interaction between an electric field and a substance is applied to an optical modulator because of its high speed, low power consumption due to driving with voltage, simplicity of its structure. In an optical modulator using LiNbO3, a Mach-Zehnder type waveguide is formed on a single-crystal LiNbO3 substrate by means of a Ti diffusion method, and it is combined with an electrode to form the optical modulator. Then, by applying voltage to the electrode, an index of refraction of the waveguide is changed, whereby ON/OFF of an optical signal can be carried out. However, since this optical modulator requires to use a single-crystal substrate, it becomes expensive. Further, since an electro-optical effect of LiNbO3 is small, the waveguide is required to be lengthened. There is a defect that the element size is very large in cm order.
An electro-optical coefficient of Pb1-xLax(ZryTi1-y)1-x/4O3 (PLZT) that is transparent ceramics is almost double-digit larger than that of LiNbO3 single crystal used in an existing optical modulator. Therefore, low cost, low power consumption and high speed can be expected by making the optical element smaller, and examination of a thin film by a sol-gel method has been made so far. For example, see “Comparison of electro-optic lead-lanthanum zirconate titanate films on crystalline and glass substrates”, K. D. Preston and G. H. Haertling, Appl. Phys. Lett. Vol. 60, No. 23, 8 Jun. 1992, 2831-2833, and “Patterning of (Pb, La)(Zr, Ti)O3 waveguides for fabricating micro-optics using wet etching and solid-phase epitaxy”, K. Nashimoto, K. Haga, M. Watanabe, S. Nakamura and E. Osakabe, Appl. Phys. Lett. Vol. 75 No. 8, 23 Aug. 1999, 1054-1056.
However, in order to form a PLZT thin film having high optical transmittance and a large electro-optical effect, it is required to be epitaxially grown. Namely, a single-crystal substrate is required as a substrate material. Therefore, there has been a defect that it is expensive because it is difficult to form it on other substrate such as a silicon system waveguide and a prolonged period of film formation process is required to form a film thickness necessary for the optical element by a sol-gel method.
Each of optoelectronics materials such as LiNbO3 and PLZT is a ferroelectric material, and its property is expressed in the case of forming a crystal structure particular to each compound. For this reason, in order to use an optoelectronics material as the optical element, it has been thought that its own single-crystal substrate is to be used or an optoelectronics material is epitaxially grown on the single-crystal substrate.
In future, realization of a nanophotonic device capable of integrating light and electronics on one chip is requested as a large innovative technology. In order to achieve this, a technique to form LSI, such as a CPU and a memory, and an active optical element, such as an optical switch, on the same substrate is required, and a technique to form a film of an optoelectronics material such as PLZT on a silicon or silica substrate with high crystallinity is requested.
On the other hand, as a new film formation technique of oxide, aerosol deposition (AD method) using a room temperature impact consolidation phenomenon has been developed. An AD method uses an impact adhesion phenomenon of an ultrafine particle material, and it is expected to achieve high film formation speed and low process temperature compared with a conventional thin film formation method. For example, see “Ceramics thin film form technique using an impact consolidation phenomenon of minute particles and ultrafine particles—low-temperature and high-speed coating by means of an aerosol deposition method—”, Jun Akedo and Maxim Lebedev, Materia, the forty-first volume, No. 7, 2002, 459-465, and “Microstructure and Electrical Properties of Lead Zirconate Titanate (Pb(Zr52/Ti48)O3) Thick Films Deposited by Aerosol Deposition Method”, Jun Akedo and Maxim Lebedev, Jpn. J. Appl. Phys. Vol. 38 (1999), Pt. 1, No. 9B, 5397-5401.
Further, since in the AD method a film property does not depend on a foundation layer, it is possible to select a substrate freely.
Japanese Patent Application Publication No. 2001-3180 discloses a low-temperature formation method of a brittle-material ultrafine particle compact using an AD method. In this method, an ultrafine particle brittle material supplied on a substrate is subjected to mechanical impact as a load, whereby the ultrafine particle brittle material is crushed and the ultrafine particle brittle materials are joined to each other or the ultrafine particle brittle materials are joined to the substrate and to each other. This achieves joining of mutual ultrafine particles, and the film with high density and high intensity is formed without applying heat.
Japanese Patent Application Publication No. 2002-235181 discloses a structure formed by an AD method. The structure is a polycrystal that has no crystal orientation, and is characterized in that there is substantially no grain boundary layer composed of a glass layer. Further, this structure is characterized in that an average crystallite diameter is 50 nm or less and compacity is 99% or more.
Examination related to formation of a thin film of an optoelectronics material having high transparency by using an AD method has been made. For example, see “Optical and electro-optical properties of Pb(Zr, Ti)O3 and (Pb, La)(Zr, Ti)O3 films prepared by aerosol deposition method”, Masafumi Nakada, Keishi Ohashi and Jun Akedo, Journal of Crystal Growth, 275 (2005), e1275-e1280. This clarifies that transmission loss of an AD film, which is a basic characteristic of an optical element, is caused by Rayleigh scattering of minute particles forming a compact and non-compact minute particles whose indices of refraction are different from those of the minute particles forming the compact.
Japanese Patent Application Publication No. 2005-181995 discloses an optical element by an AD method, an integrated optics device, an optical information transmission system and a method of manufacturing the same. In this method, a compact is formed by an impact consolidation phenomenon in which an ultrafine particle brittle material supplied on a substrate is subjected to mechanical impact force as a load, and the ultrafine particle brittle material is crushed and joined to each other, whereby an optical element is obtained. This optical element includes a portion having an index of refraction different from that of main formation of the compact, such as a pore (hole) and a heterogenous phase, and is characterized in that there is a relationship between an average radius d (nm) of the portion and a wavelength λ (nm) of light that propagates through the compact as d6/λ4<4×10−5 (nm2).
Transmission loss of a thin film formed by an AD method is composed of two components, optical scattering and optical absorption. As described above, a property of transmission loss due to optical scattering of the thin film formed by the AD method and a reduction method were revealed, but a method of reducing the optical absorption remains unrevealed.